In recent years in both this country and abroad, there has been a resurgence of acute, often life threatening infections due to Streptococcus pyogenes (group A streptococcus). Many patients experience symptoms mimicking this associated with staphylococcal toxic shock syndrome, and the designation streptococcal toxic shock syndrome (STSS) has been assigned to these invasive streptococcal infections. There is strong epidemiologic evidence implicating streptococcal pyrogenic exotoxin A (SpeA) in the pathogenesis of STSS. SpeA is a bacterial superantigen that is capable, in combination with class II major histocompatibility molecules, of activating a large fraction of T cells. The pathology of STSS and other bacterial superantigen mediated disease is believed to result from the massive and unregulated release of bioactive cytokines from the activated T cells. SpeA and other bacterial (and viral) proteins have been termed superantigens due to their unique mechanisms of interacting with the class II MHC expressing antigen-presenting cells and the T lymphocytes. Superantigens bind to class II MHC as intact molecules at sites distinct from the antigen-presenting groove. In addition to binding the class II MHC molecule, superantigens interact with the T cell receptor (TCR) in regions encoded by the V-gene segments. Each superantigen activates a specific set of V-beta chain-encoding T cell, and thus is able to activate a much larger percentage of the T cell population than conventional peptide antigens. The goals of this grant proposal are to examine in detail the interaction between SpeA and the human TCR. The amino acid residues of SpeA needed for a productive TCR interaction will be defined. V-beta chains amino acids residues needed for a productive interaction with SpeA will be identified. These studies will include measurements of the affinities of the toxin- V-beta interactions. In addition, the X ray crystal structure of the toxin, mutant and allelic toxin forms, as well as the toxin completed with a human V-beta chain will be determined. These data will help characterize the interaction needed for activation of a T cell by the superantigen. In addition, the data obtained here are the first step in the design and development of compounds, such as TCR specific peptides, to interfere with the SpeA-TCR interaction, and in turn prevent SpeA from acting as a superantigen. It is possible that components that can specifically block the superantigenic capabilities of SpeA might prove useful as treatments to ameliorate and possibly prevent STSS in an infected individual.